Many applications require knowledge of the identity and concentration of trace anions in water or other solutions. For samples of limited volume, rapid, simultaneous identification and quantification of such anions becomes extraordinarily difficult as the background ion concentrations enter the part per billion (ppb) range. As the amount of the anion of interest decreases in the sample, the difficulty of detecting and quantifying the anion in solution increases. Many current techniques require the anion to be present in concentrations of at least a ppb for quantitative detection, but such concentrations can be unattainable in many applications (e.g., radiotracer synthesis). Additional difficulties arise when the anion of interest is in a sample with competing anions at similar or higher concentrations. In particular, detecting the presence and concentration of a specific anion of interest in the presence of other anions of similar reactivity can be difficult. For example, low concentrations of chloride ion may be precipitated from an aqueous solution using silver ion, but any bromide or fluoride ions present will also precipitate as insoluble silver bromide and silver fluoride salts.
As another example, determining the amount and purity of 18F incorporated during the synthesis of positron emission tomography (PET) radiotracers can be difficult. Such detection problems arise because background 19F-fluoride ion concentrations in the 18O-labeled water target may substantially exceed the amount of 18F-labeled fluoride in the radiotracer sample. When the background concentration of 19F-fluoride ion is large, the synthesized radiotracer can be contaminated with the non-radioactive analogue. In such cases, the radiochemical purity and suitability of the radiotracer for PET imaging may be compromised.